These elders of the forest do many things that smaller, younger trees cannot; for example, providing homes for many types of animals, providing space for other plants to grow in tropical rainforests and producing large amounts of seeds that serve as food for other animals and replenish tree populations, according to the study, published today (Dec. 6) in the journal Science.
Old trees also store an enormous amount of carbon and continue to sequester it as they grow, even in their old age, said study co-author David Lindenmayer, a researcher at Australian National University. One study published in PLoS ONE in May found that although big trees, with a diameter of more than 3 feet (1 meter) at chest height, made up only 1 percent of trees in a study plot in California's Yosemite National Park, they accounted for 50 percent of the area's biomass.
Another study found that huge mountain ash trees in southern Australia and Tasmania provide homes for more than 40 species of animals, which live in cavities in the old trees, Lindenmayer said. Smaller trees provide homes for far fewer animals.
The decline in these trees is happening globally. "Large old trees are declining rapidly in all kinds of ecosystems worldwide - forests, rainforests, boreal forests, woodlands, agricultural areas, cities and savannahs," Lindenmayer told OurAmazingPlanet.
While the loss of these sylvan elders is sometimes obvious, in the case of forest fires or clear-cutting, their disappearance is usually less apparent, said Nate Stephenson, an ecologist with the Western Ecological Research Center in Three Rivers, Calif. "Losses of big, old trees can take place over decades, generally too slowly for people to notice, become alarmed about and take actions to correct," said Stephenson, who wasn't involved in the Science study. "The next generation may not know that big old trees were once common in the nearby forest."
Why the decline?
Big trees are in decline for a number of reasons, including logging and clearing of land for agriculture, introduction of non-native insects or pathogens (an example being chestnut blight), past management practices (for example, fire exclusion that has led to denser forests, which can be more vulnerable to insect outbreaks and severe fires), air pollution and climatic changes, Stephenson said.
The decline in any one place, though, is specific to the area, Lindenmayer said. "It might be elephants plus fire plus fungi in [South Africa's] Kruger National Park, versus fire plus logging plus climate change in the wet forests of Victoria," in Australia, he said. "But the problem manifests in broadly the same way in all systems: rapid loss of existing large old trees and often a failure to recruit new big trees, leading to a massive vacuum."
Though almost none of the "big tree" species are in danger of going extinct, the largest individuals could become very rare, said James Lutz, a researcher at the University of Washington, who wasn't involved in the study.
To prevent losing more forest giants, people need to protect individual large trees and places where they are more likely to grow, Lindenmayer said. It's also important that land managers realize the importance of big old trees. "Many managers have no idea about this," he said.
"We all know that big animals like whales, tigers are in trouble - now we have seen that big trees are too," he said. The problem is these trees take centuries to get big, he said.
Big trees are important to people, as well, Lutz said. "They are majestic, though they were once as all the other saplings," he said. "By preserving them, we preserve our heritage and our hope and maybe a bit of ourselves as well."